A turboprop engine is a propulsion means that is used in particular on certain aircraft, and comprises a gas turbine and a propeller. The propellers of most turboprop engines nowadays are propellers comprising variable-pitch blades. These are also referred to as variable-pitch propellers. A propeller having variable-pitch blades is equipped with a mechanism allowing the pitch angle of the blades to be adjusted, i.e. the angle between the reference chord of a blade and its rotational plane. Controlling the pitch angle of the blades makes it possible to adjust the properties of the propeller so as to optimise the performance of the turboprop engine within a large flight envelope. For example, for the take-off and climbing phases of the aircraft, the aim is generally to provide a significant pitch so as to maintain a reasonable propeller speed at high levels of power. For cruising flight phases, the aim is generally to provide a lower pitch in order to maintain a reasonable propeller speed at lower levels of power. During landing, the aim is generally to provide a negative pitch in order to decelerate the aircraft and reduce its braking distance. It is therefore necessary to provide a control device that allows the pitch of the blades of a propeller to be modified during flight.
As a general rule, propellers comprising variable-pitch blades are controlled in two operating modes, depending on the flight phases of the aircraft. For low engine speeds, the pitch of the propeller is controlled directly. Throughout the text, this mode is referred to as the “beta mode”. For higher engine speeds, the speed of the propeller is controlled to an optimal operating point. Throughout the text, this mode is referred to as the “speed mode”.
It is therefore necessary to provide both a control system for the beta mode which slaves the pitch to a pitch setpoint, and a control system for the speed mode which slaves the propeller speed to a speed setpoint. These pitch and speed setpoints are either fixed or are provided by means of exterior mechanical cables or electrical signals.
Currently, there are three distinct groups of systems for controlling a propeller: electrical control, electro-hydraulic control by servo valves and hydromechanical control.
In practice, electrical control and control by servo valves cannot be used for all applications, either because of the total weight of the system (this is the case in particular for control by servo valves), the costs incurred (this is the case in particular for electrical control or control by servo valves) or because they are not advanced enough (this is the case in particular for electrical control).
The principle of hydromechanical control is to apply oil pressure to a piston rigidly connected to a mechanism that drives the blades in rotation about their axis. The piston is for example housed in a cylinder in the propeller hub, and defines one or two pressure chambers. Reference is made to a double-action propeller when the pressure can be exerted from either side of the piston. Reference is made to a single-action propeller when the pressure can only be exerted from one side of the piston. In this case, the piston is returned by means of a spring and balance weights arranged at the root of each blade.
A device for controlling a propeller having variable-pitch blades comprises, in a known manner, a first piece of hydromechanical equipment for slaving the pitch of the blades of the propeller to a pitch setpoint, and a second piece of hydromechanical equipment for slaving the rotational speed of the propeller to a speed setpoint.
The first piece of hydromechanical equipment for slaving the pitch generally uses a slide actuated both by the propeller (which makes it possible to obtain a copy of the angle of the blades) and a mechanical connecting link actuated by the pilot in order to provide the pitch setpoint.
The second piece of hydromechanical equipment for slaving the speed uses a speed controller based on rotating balance weights.
Therefore, in speed mode, the rotating balance weight system, which is rotated by the engine reduction gear at a speed proportional to that of the propeller, drives a slide which controls the piston of the propeller, and therefore its pitch, either directly or by means of a hydraulic amplifier. The calibration of the return spring of the balance weights defines the speed setpoint. This speed setpoint can be modified, for example by adjusting the calibration of the spring by means of a mechanical cable or an all-or-nothing electrovalve. In beta mode, the hydraulic slides that are mechanically connected to the pitch setpoint and the pitch copy make it possible to slave the pitch to the intended setpoint. The pitch setpoint is mechanically transmitted to the slide by the mechanical connecting link controlled from the cockpit.
This solution has two drawbacks. First, the control of the speed setpoint by action on the return spring is either binary (2 selectable speeds) or is implemented by a mechanical connecting link that does not allow control by means of an electronic device such as an engine control computer. Second, the pitch setpoint is controlled by a mechanical connecting link connected to the cockpit controls, and therefore does not allow the pitch to be directly controlled by means of an electronic control computer.
The inventors have therefore sought to improve the principle of hydromechanical control of a propeller having variable-pitch blades.